The promotion is running through October, with some titles selling at 99p throughout the month, while others are offered at the cut-rate price for 24 hours only.

George R R Martin's A Game of Thrones (HarperCollins), J K Rowling's The Casual Vacancy (Little, Brown) and Gillian Flynn's Gone Girl (Phoenix) have been included in the promotion, all of them price-matched on Amazon.

Yesterday (8th October), nine Ian Rankin novels (Orion) were being sold at 99p on the Sainsbury's site, with Amazon following suit today.

Tim Lennox, m.d. of eBooks by Sainsbury's, told The Bookseller that the offer was proving to be a success, with the "best day's sales since we started" coming when A Game of Thrones was offered for 99p.

He said: "The books on offer appeal to our customer base and are on offer for a period of time. The offer changes every day so we could keep it new and innovative. Competitors will match what we are doing, that was always going to be expected. This way, we stay on the front foot."

The supermarket chain is also promoting its e-book service in store across its non-book products, with advertisements appearing on the back of its own-brand cereal boxes offering a free e-book from James Patterson or Jacqueline Wilson. Lennox said: "It is also encouraging children to read and I know the authors are very keen on doing that."